The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Cross-linking agents, such as borates, titanates or zirconates, are usually incorporated into the fluid composition to control viscosity.
Typically, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through permeable zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.
Difficulties in oil recovery due to thief zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linking agent into a subterranean formation under conditions where the polymer will be cross-linked to produce a gel, thus reducing permeability of the subterranean formation to the driving fluid (gas, water, etc.). Polysaccharide- or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium, and boron based compounds are used in these enhanced oil recovery applications. Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of zones in subterranean formation, are now being used in hotter and deeper wells under a variety of temperature and pH conditions. In these operations the rate of cross-linking is critical to the successful generation of viscosity.
Boron-based compounds are typically used as cross-linkers in fracturing fluids utilized in low to mid temperature wells (150-250° F., 66-121° C.). Cross-linking takes place immediately on mixing of the boron compound with the polymer base-gel. A pH of 10 or greater is required to initiate cross-linking with boron-based cross-linkers. Because boron cross-linked gels are not shear sensitive, they can be used, even though they cross-link at or near the surface.
Existing delayed zirconium-based cross-linkers, based on triethanolamine or hydroxyalkylated ethylenediamine have been designed to initiate cross-linking in the wellbore. Therefore, they are ineffective at generating viscosity under mild surface temperature conditions. The gels are also shear sensitive and require higher horsepower (energy consumption) to pump.
The need exists in some fracturing fluid applications to generate an initial viscosity at the surface, followed by a delayed viscosity generation, once the fluid is subjected to higher down-hole temperatures. In the case of mid-high temperature wells (250-300° F., 121-149° C.), a 3-8 minute delay in cross-linking is preferred. For deeper, higher temperature wells (300-400 ° F., 149-204° C.), it may be necessary to have cross-link times from 5-10 minutes.
Current technology involves using a borate-ion-generating-material in combination with a delayed zirconate cross-linker to accomplish both surface and delayed viscosity development. However, borate/zirconate cross-linking compositions suffer from disadvantages, such as poor shelf stability, insufficient viscosity generation and undesirable cross-linking rates.
U.S. Pat. No. 4,686,052 discloses a cross-linker comprising an organic zirconate stabilized with triethanolamine, optionally to which borax may be added. The cross-linker mixture with borax has extremely long cross-linking time and low viscosity development.
There is a need for a borozirconate cross-linker which is stable on extended storage, is capable of generating excellent viscosity in the desired 5-10 minute range for use in higher temperature wells, and which can be used in place of existing delayed zirconate cross-linkers in areas where an initial surface viscosity development is required, or in place of delayed borate cross-linkers, which generally have limited temperature application. The present invention meets these needs.